Anisotropic monotectic alloys and process for making the same



March 2s, 1969 F. n. LEMKEY 3,434,827

ANISOTHOPIC MONOTECTIC ALLOYS AND PROCESS FOR MAKING THE SAME Filed July 16, 1965 sheet I of `:s

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March 25, 1969 F. D. L EMKE'Y ANISOTROPIC MONOTECTIC ALLOYS AND PROCESS FOR MAKING THE SAME Sheet of 3 Filed July 16, 1965 F/G. Z

United States Patent Office U.S. Cl. 75--134 14 Claims ABSTRACT oF THE DIsCLosUIm Monotectic compositions are solidified to form a body wherein one of the porducts of solidification forms as filaments embedded in a matrix of a second product of solidiication, the filaments being arranged in parallel, spaced relationship and having a melting point lower than that of the matrix material. Because of the difference in melting points of the two products of solidification, the body may Ibe reheated to a temperature at which the filaments melt while the matrix remains solid. The molten material may then be selectively removed to form a body with uniaxial porosity or other useful properties.

This invention relates in general to unidirectionallysolidified alloys and compositions characterized by a high degree of microsturctural regularity and continuity, and to methods for producing such alloys. More particularly, this invention contemplates the for-mation of anisotropic alloy compositions from montectic alloys wherein the lower melting product of solidification forms as rods or filaments embedded in a higher melting `solidilication product. It further contemplates the production of porous matrix materials from the unidirectionally solidified ingot, the pores of which are of uniform size and shape and are substantially straight, independent and parallel with respect to one another.

It is known that certain polyphase alloys having microstructures which consist predominantly of very fine, three dimensional lamellae oriented substantially parallel to a common direction may be produced by unidirectional solidilication techniques. These microstructures are generally formed from a eutectic composition or alloy, as discussed in the patent to Kraft, 3,124,452, a eutectic composition being defined as one in which two or more types of crystals freeze simultaneously at a fixed temperature, called the eutectic temperature, upon cooling from the liquid state. In those eutectic compositions which can be caused to grow in the form of three-dimensional lamellae, when one of the phases in a two phase alloy is caused to grow as parallel rod-like lamellae throughout a large specimen, the other phase will form a matrix in which the rods are embedded.

It is an object of hte present invention to produce alloys having a microstructure which consists perdominantly of a rod-like product of solidilication embedded in a matrix product, the two products being characterized by different physical properties including different melting points.

It is a further object of this invention to produce alloys having a microstructure in which the rod-lke material is regularly and unformly dsitributed in an ordered relationship within the body of the matrix.

An additional object is the provision of anisotropic alloy compositions in which the rod-like material is in the form of filaments of uniform configuration, size and orientation, each filament being structurally independent of each other filament over a substantial length.

Another object is to provide a method of producing porous matrix materials from such alloys, the pores comprising a plurailty of uniform, parallel and independent 3,434,827 Patented Mar. 25, 1969 channels of substantial length regularly spaced throughout the matrix.

These and other objects and advantages of this invention will be evident from the following description or from practice of this invention as taught herein.

It has been demonstrated that by the monotectic reaction, in which one liquid phase (LI) decomposes with decreasing temperature into a solid phase (a) and a new liquid phase (LH), compositions can be unidirectionally solidified to yield microstructures in which the products of solidification of the LII phase are present in the form of uniformly thin rods, all of which are essentially parallel and regularly spaced over `considerable distances. Since the LII phase freezes at a lower temperature (TE) than that of the matrix (a phase), it is possible to reheat the solidilied structure above the freezing temperature of the rods and yform liquid channels lwithin the matrix. The liquid may then be forced out of the channels by imposing a suitable pressure differential across the channels in a direction parallel to their longitudinal axes.

The present invention may best be described by reference to the following drawings, in which:

FIGURE l is a partial equilibrium diagram of the system Sb-S which undergoes the monotectic reaction.

FIGURE 2 is a photomicrograph of a section of the unidirectionally solidified structure of the montectic system Sb-S taken normal to the rod axis.

FIGURE 3 is a photomicrograph of the monotectic system Sb-S taken parallel to the axis of the rods.

FIGURE 4 is an enlarged photomicrograph of the Sn-S monotectic system illustrating the uniformity of rod configuration and spacing in the material.

FIGURE 5 is a schematic representation of the steadystate growth of a rod-like monotectic.

FIGURE 6 is a diagram illustrating the effect of the rod growth rate on the inter-rod spacing.

FIGURE 7 is a sketch of the solidiiication apparatus `used in the formation of the materials of the present invention.

FIGURE 8 is a schematic of a liquid blower usable lin the formation of porous matrices according to this invention.

The monotectic reaction has much in common with the eutectic reaction involving the simultaneous production of two or more phases in fine states of subdivision. Freezing proceeds by nucleation and subsequent growth and it is diflicult to distinguish between these two stages from the microstructures of the as-cast monotectic alloys.

In the Sb-S system to which most of the discussion will be directed for the sake of brevity, the monotectic invariant occurs at 1.5 weight percent (5.5 atomic percent) sulfur. The phase diagram indicating the equilibria involved is illuistrated in FIGURE 1. When phase LI, rich in antimony, is cooled through the three phase reaction isotherm (615 C.), solid antimony and phase Ln, a liquid rich in sulfur, simultaneously appear. Upon further cooling to a temperature of 520 C., phase LII undergoes a eutectic reaction in which Sb2S3 and antimony are simultaneously crystallized.

The postulated crystal Agrowth yielding the rod-like structure adjacent the solid-liquid interface in the Sb-S system upon controlled solidication is illustrated schematically in FIGURE 5. As illustrated, the matrix is antimony and the rod-like structure is the Sb2S3-Sb eutectic. It is theorized that two liquid-solid interfaces advance as solidification proceeds, one at an axial position in the solidification apparatus whereat the temperature corresponds to the monotectic iotherm (TM) and the other whereat the temperature corresponds to the eutectic isotherm (TE).

It has been discovered that most monotectics investigated do not entrap the liquid formed (LH) at the monotectics isotherms as in the case with the above-described Sb-S system, but instead the leading vliquid-solid advances the phase LH and it eventually fsolidies at the grain boundaries. Others have investigated the interaction between the liquid particles and the solid-liquid interface and have found that, for a given type of particle, solid or liquid, there exists a critical interface propagation rate below which particles may be rejected or pushed aside and above which they are entrapped. No such critical velocity has been found in connection with the Sb-S system, however.

In the controlled monotectic specimens studied it has been observed that both phases extend in a direction very nearly parallel to that corresponding to the direction of soliditication. However, when a perturbation is introduced into an otherwise steady structural growth rate during the unidirectional solidiiication, these perturbations are reflected as banding on the structure and trace the position of the macroscopic liquid-solid interface which is slightly convex in some cases toward the liqud.

TABLE 1 Alloy System Matrix Rod Structure The quantitative variation of interphase spacing as a function of solidification rate is illustrated in FIGURE 6 and was studied by measuring the inter-rod distances between the nearest neighbors in a plane perpendicular to the rod axis. The data accumulated indicates that radial solute diffusion in .he liquid ahead of the advancing interface plays an important role in controlling the interrod spacing.

It has similarly been established that an increase in the rate of the planar solid-liquid interface movement decreases both the spacing and the diameter of the rods. Therefore, by judicious selection of the interface propagation rate the number of filaments per unit area may be varied. Similarly, by means of intermediate changes in the solidication velocity matrices containing lilaments whose diameter varies with their length may be produced,

and additionally, by selectively removing the aligned phase or phases, matrices exhibiting multiple porosity may be achieved.

Although the mechanism of this invention is described with specific reference to the true monotectic compositions of the various alloys, it is possible to produce the structures described with considerable deviation from the true monotectic composition. When such a deviation is permitted, the product will usually still be characterized by the desired ordered filamentary microstructure. However, the product will also be characterized by the presence of relatively larger crystals of one of the elements or alloys selectively distributed throughout the microstructure. These are normally referred to as primary crystals since they are the irst crystals to solidify as the temperature of the melt is lowered in the soliditication process. This mechanism is well understood by those conversant in the art.

It is also contemplated that minor additions of specilic impurities may be advantageously made to the melt prior to the controlled soliditication to impart specific properties to either the matrix structure or the filamentary material or to further enhance properties otherwise inherent therein. For example, an intentionally added impurity which exhibits solid solubility with the matrix material may be utilized to alter the electrical properties of the matrix to achieve a specific electrical resistivity, or to alter its magnetic properties.

Throughout this description and in the claims which follow, it has been convenient to describe the lower melting product of solidification in the unidirectionally solidified ingot as rod-like, rods or filaments. However, it will be evident that the cross-sectional shape of the individual filaments is not critical. T he filaments need not be circular in cross-section nor `is it necessary that the ingot or rods be in the form of a straight cylinder. Nor is it necessary that the length of the rod exceed its diameter although such will normally be the case. The terms rods and filaments then will be understood to include a wide variety of solidification product configurations in the context as herein intended.

Alloy specimens were prepared by mixing the individual components in a quantity approximately corresponding to the monotectic composition. The mixture was sealed in an evacuated quartz tube and the tube was heated to and held at a tempreature of approximately 450 C. (in the case of the Sb-S system) for 24 hours before any controlled solidification was undertaken. It will be understood, of course, that in this or other systems different temperatures and different reaction times will be satisfactory or necessary, depending upon the physical and chemical characteristics of the elements involved and the equipment utilized. The precise parameters specied for the reaction step are relatively immaterial as long as they are sufficient to effect the desired reaction between the individual components. It should be emphasized, however, that considerable care must be exercised during the reaction process for safety reasons, particularly when the individual components are characterized by widely divergent melting and boiling points. In the Sb-S system for example, the sulfur melts at about 119 C. and boils at 445 C. while the antimony melts at 630 C. During the process of reacting these two elements, therefore, it will be seen necessary that due consideration be given to the potential pressure buildup in the reaction vessel due to vaporization of the sulfur as the temperature is increased. In the Sb-S system, the reaction temperature was selected to yield a slightly superheated liquid sulfur and yet to permit the use of conventional glassware reaction equipment. The selection of the parameters necessary to react the various components when other materials are utilized is well within the knowledge of those skilled in the art.

It is, of course, possible to effect the requisite reaction between components directly in the crucible of the solidilication apparatus discussed below. However, because the speed of the reaction is determined, at least in part, by the developed area at the interface between the liquid sulfur and the solid antimony the reaction was normally effected in a separate step with the axis of the finished specimen oriented in a generally horizontal direction to provide for the desired increased contact area between solid and liquid.

After the reaction went to completion, the specimen was removed from the quartz tube and vertically positioned within a unidirectional soliditication furnace of the form illustrated schematically in FIGURE 7. This apparatus comprises a hollow containment tube 2 partially surrounded by an induction coil 4 which is suitably attached to a power source, not shown. Slidably mounted within the containment tube 2 is a graphite Crucible 6 wherein the specimen 8 is positioned, the annular space l0 surrounding the crucible normally being filled with argon or some other protective atmosphere to protect the crucible and from atmospheric contamination during the heating process. The crucible `6 is counterbored at its lower end and threaded to a water-cooled brass plug 12, the plug being attached to a variable drive mechanism (not shown) whereby the crucible may be drawn down through the induction coil at a predetermined rate. Thermocouple 14, shown projecting upwardly through the bottom of the crucible cavity wherein the specimen is carried, is used to sense the temperature of the speci men and establish that the specimen lis completely liquified before withdrawal of the crucible from the furnace and directional solidification is commenced.

After the crucible had been positioned in the crucible cavity, power was applied to the induction coil and the temperature of the specimen was raised to a temperature in excess of the monotectic isotherm. Preferably the entire specimen is melted before a resolidification is attempted, although it was quite common in the tests conducted to initiate the process with a small portion of the specimen remaining unreacted where is abutted the water-cooled brass plug 12. When the specimen had been completely melted,.cooling water was admitted to the brass plug and unidirectional solidification was effected by slowly withdrawing the crucible from the furnace.

The freezing rate over the central portion of the ingot was varied generally from 0.8 to 8.0 cm./hr., this range being defined by the physical limitations of the equipment being utilized. The imposed temperature gradient in the liquid was approximately 70 C./in. in the vicinity of the monotectic reaction, and the ingots were unidirectionally solidified usually to a length of approximately 5 inches, although this length was a purely arbitrary selection. The temperature gradient will be understood to mean the change in temperature in the liquid per inch of length in the liquid immediately in front of the advancing solid monotectic interface. In every ingot fabricated a fine-grained structure was produced at the end of the specimen which was first crystallized and` from this polycrystalline region evolevd the large grains elongated in the direction of growth.

YWhile it is preferred to both completely liquefy the monotectic specimen prior to resolidification and further to initiate the solidification at the bottom of the specimen, this recommended technique is not necessary as long as a stable solid-liquid interface is established and a predetermined grain growth rate program is maintained. Similarly, although the solidification was undertaken with the apparatus mounted vertically, equally good results will be obtained in` modified equipment oriented in directions other than the vertical.

As has been previously noted and discussed in connection with FIGURES 2, 3 and 4, the solidified product is characterized by a high degree of microstructural regularity and continuity. The structure consists of a plurality of rods or filaments lof consistent cross-sectional area and configuration regularly disposed in a matrix material of a different composition. Further,` the rods are all parallel and independent substantially throughout the length of the matrix structure. It will vbe further noted from the various systems, cited for illustrative purposes in Table 1, that `through the practice of this invention it is possible to form a structure wherein the rods are electrically conductive `and the matrix is semi-conductive or, conversely, to produce a structure wherein the rods are semi-conductors emedded in a conductive matrix material. For example, in the Sb-S system of FIGURE 2 and 3, the matrix is antimony and, accordingly, an electrical conductor. On the other hand, in the Sn-S system of FIGURE 4, the matrix is tin sulfide, a semi-conductor. The usefulness of the products of this invention in the electrical and electronic industries will be obvious.

It is further suggested that through the practice of this invention by selectively dissolving the matrix material from the periphery of the rods in the solidified ingot, it is possible to produce very thin fibers of essentially uniform cross section and length.

Since a structure is produced wherein the rod-like material has a lower melting point (TE) than the matrix material (TM), it is possible to reheat 'the specimen to a temperature intermediate the two melting points and from liquid channels within the solid matrix. The liquid may be forced out of the channels by applying a suitable pressure differential across the specimen in a direction parallel to the channels sufiicient to overcome the surface tension of the liquid. Once the critical pressure differential has been established for breakaway and the liquid removed, the fiow of gas through the specimen together with the pressure drop will define the efiiciency of the liquid removal, the flow rate being related to the pressure drop across the specimen and the number of open channels `for Hagen-Poiseville ow. In this manner it is possible therefore to form a plate which is characterized by relatively uniform and unidirectional porosity. Further, in addition to the use of articles formed in this manner as porous materials, these structures may have other elements or materials with favorable melting points and surface tensions backfilled into the channels, yielding articles of potentially useful and unique properties. A particularly useful example of this type would comprise a refractory matrix with a uniformly dispersed low melting phase backlled into the pores, resulting in a structure which will ablate uniformly in one direction in a high temperature environment.

A schematic of liquid -blower illustrated in FIGURE 8 has been utilized to demonstrate the removal of liquid from the channels in thin discs cut from unidirectionally solidified ingots. The blower comprises a quartz envelope 20 surrounded by an induction heating coil 22. The specimen 24 is suitably positioned within the quartz envelope internal of the induction coils and between two flanged conduits 26- and 28 through which high pressure inert gas is circulated. A pressure gage 30` communicating with the flow passage within the conduit 26V and a second pressure gage 32 communicating with the fiow passage within conduit 28 are utilized to establish the appropriate pressure differential across the specimen. A owmeter 34 is provided to sense the flow rate of inert gas within the conduit 28. The selection of appropriate materials usable in this apparatus is within the state of the art and a quartz envelope surrounding conduit materials composed of stainless steel have been utilized. It may be found advantageous to circulate an inert gas such as argon in the annulus 36 formed between the envelope and the conduits to prevent oxidation of the conduit material and contamination of the specimen at the processing temperatures.

Although the use of a gas pressure differential has been suggested as a possible means of preferentially removing the rod-like material from the solidified structure to form pores therein, it will be understood that other means are contemplated. For example, in a given system, the obvious difference in vapor pressures between the individual products of solidification may suggest that n controlled ablation or evaporation technique may advantageously be utilized. Further, depending again on the chemical nature of the system involved, it may be preferable to selectively dissolve the rod material and form the porous structure in this manner.

I claim:

1. An antimony-sulfur monotectic composition characterized by a microstructure which comprises a matrix consisting essentially of antimony and a plurality of substantially uniform and parallel filaments embedded in said matrix in uniform array, said filaments consisting essentially of an antimony-antimony trisulfide eutectic.

2. A tin-sulfur monotectic composition characterized by a microstructure which comprises a matrix consisting essentially of stannous sulfide and a plurality of substantially uniform and parallel filaments embedded in said matrix in uniform array, said filaments consisting essentially of tin-stannous sulfide eutectic.

3. A silver-sulfur monotectic composition characterized by a microstructure which comprises a matrix consisting essentially of silver and a plurality of substantially uniform and parallel filaments embedded in said matrix in uniform array, said filaments consisting essentially of a silver-silver sulfide eutectic.

4. A copper-sulfur monotectic composition characterized by a microstructure which comprises a matrix consisting essentially of cuprous sulfide and a plurality of substantially uniform and parallel filaments embedded in said matrix in uniform array, said filaments consisting essentially of a copper-cuprous sulfide eutectic.

5. An article of manufacture characterized by uniaxial porosity formed from that class of monotectics in which one of the products of solidification forms as laments embedded in a second product of solidification, said filaments having a lower melting point than said second product of solidification, comprising a matrix consisting of said second product of solidification having uniformly disposed therein a plurality of substantially straight, independent and parallel channels, said channels having a relatively uniform size and configuration and being formed by the selective removal of said filaments.

6. An anisotropic body formed from that class of monotectics in which one of the products of solidification forms as filaments embedded in a second product of solidification, said filaments having a lower melting point than said second product of solidification, comprising a matrix consisting of said second product of solidification having uniformly disposed therein a plurality of substantially straight, independent and parallel channels, said channels having a relatively uniform size and configuration and being formed by the selective removal of said filaments, and a third material backfilled into said channels, said third material having a melting point lower than said second product of solidification.

7. The method of forming anisotropic alloy compositions comprising the steps of:

forming a monotectic composition selected from that group of monotectics in which one of the products of solidification forms as filaments embedded in a second product of solidification, the filaments being substantially uniform in size and regularly spaced in parallel relationship over a substantial length,

heating the composition to a temperature above the monotectic temperature, and

unidirectionally solidifying the melt to cause filament growth.

8. The method of claim 7 wherein:

the monotectic composition is selected fromA that group of monotectic systems consisting of Sb-S, SnS, Ag-S, Cu-S, In-S and Tl-S. 9. The method of forming a body of uniaxial porosity comprising the steps of:

forming a monotectic composition selected from that group of monotectics in which one of the products of solidification forms as filaments embedded in a second product of solidification, said filaments being substantially uniform in size and regularly spaced in parallel relationship over a substantial length,

heating the composition to a temperature above the monotectic temperature,

unidirectionally solidifying the melt to cause filament growth, and selectively removing the filament material to form a porous body. 10. The method of claim 9 wherein: the monotectic composition is selected from that group of monotectic systems consisting of Sb-S, Sn-S, Ag-S, Cu-S, In-S and Tl-S. 11. The method of claim 10 wherein the selective -removal of the filament material includes the steps of:

reheating the body to a temperature intermediate the melting points of the first and second products of solidification whereby the filaments are liquefied, and imposing a pressure differential across said body sufficient to eject the liquefied filament material.

12. The method of claim lwherein the selective removal of the filament material includes the steps of:

reheating the body to a temperature intermediate the melting points of the first and second products of solidication whereby the filaments are liquefied, and imposing a vacuum on the body whereby the filament material is selectively evaporated. 13. The method of forming articles having anisotropic properties comprising the steps of forming a monotectic composition selected from that group of monotectics in which one of the products of solidification forms a filament embedded in a second product of solidification, said filaments being substantially uniform in size and regularly spaced over` a substantial length, heating the composition to a temperature above the monotectic temperature, unidirectionally solidifying the melt to cause filament growth, selectively removing the filament material to form a porous body, and backfilling the porous of the porous body with a third material having a melting point below that of the second product of solidification. 14, The method of forming fibers of uniform shape, diameter and length comprising the steps of:

forming a monotectic composition from that group of monotectics in which one of the products of solidification forms as filaments embedded in a second product of solidification, said filaments being substantially uniform in size and regularly spaced in parallel relationship over a substantial length,

heating the composition to a temperature above the monotectic temperature,

unidirectionally solidifying the melt to cause filament growth, and

selectively removing the second product of solidification to release the individual filaments.

References Cited UNITED STATES PATENTS 3,124,452 3/1964 Kraft 75-134 3,226,225 12/1965 Weiss et al 75--134 RICHARD O. DEAN, Primary Examiner.

Us. ci. XR. 

